Research Article Emissions of SO from a Coal-Fired ...downloads.hindawi.com/journals/isrn/2013/514751.pdfHeterogeneous Catalysis of SO 2 on Bed Particles and Heat Transfer Surfaces

Hindawi Publishing CorporationISRN Environmental ChemistryVolume 2013, Article ID 514751, 7 pageshttp://dx.doi.org/10.1155/2013/514751

Research ArticleEmissions of SO3 from a Coal-Fired Fluidized Bed underNormal and Staged Combustion

Wasi Z. Khan,1 Bernard M. Gibbs,2 and Assem Ayaganova1

1 Department of Chemical Engineering, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan2Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK

Correspondence should be addressed to Wasi Z. Khan; [email protected]

Received 26 February 2013; Accepted 28 March 2013

Academic Editors: N. Fontanals, F. Long, and K. L. Smalling

Copyright © 2013 Wasi Z. Khan et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This paper reports the measurements of SO3emissions with and without limestone under unstaged and staged fluidized-bed

combustion, carried out on a 0.3 × 0.3m2 and 2m high stainless-steel combustor at atmospheric pressure. The secondary air wasinjected 100 cm above the distributor. SO

3emissions were monitored for staging levels of 85 : 15, 70 : 30, and 60 : 40, equivalent to a

primary air/coal ratio (PACR) of ∼0.86, 0.75, and 0.67. Experiments were carried out at 0%–60% excess air level, 1-2m/s fluidizingvelocity, 800–850∘C bed temperature, and 20–30 cm bed height. During unstaged combustion runs, SO

3emissions were monitored

for awide range ofCa/S ratios from0.5 to 13.However, for the staged combustion runs, theCa/S ratiowas fixed at 3. SO3was retained

to a lesser extent than SO2, suggesting that SO

2reacts preferentially with CaO and that SO

3is involved in the sulphation process

to a lesser degree. The SO3emissions were found to be affected by excess air, whereas the fluidizing velocity and bed temperature

had little effect. SO3was depressed on the addition of limestone during both the staged and unstaged operations, and the extent of

the reduction was higher under staged combustion.

1. Introduction

The presence of SO3in flue gas corrodes the equipment

and ducts of combustion system and therefore needs tobe removed [1]. In order to control emissions of SO

3,

more studies on its formation and dissociation are requiredunder air-fired and oxy-fired combustion conditions. Thesimulation study of Zheng and Furimsky [2] shows that SO

3

emissions would be unaffected during oxy-fuel combustion,being governed only by oxygen concentration. The kineticsof reactions occurring in the combustor were studied byBurdett et al. [3] using a TGA microbalance. They proposedthe following mechanisms for the formation of SO

3.

1.1. SO2/SO3Homogeneous Gas Phase Reaction. SO

2may be

oxidized to SO3by two reactions:

SO2+O2→ SO

3+O (1)

M + SO2+O → SO

3+M (2)

where M is a chaperon third body molecule. The largetemperature dependence of reactions (1) and (2) ensuresthat the rate of production falls rapidly with decreasing gastemperature, and, in fact, 90%–95% of SO

3is formed in the

bed and freeboard and the remaining 5%–10% in the regionbetween the freeboard and sampling point. SO

3increases

sharply with temperature, but the homogeneous reactioncannot account for all the SO

3produced.

1.2. Heterogeneous Catalysis of SO2on Bed Particles and

Heat Transfer Surfaces. In a coal burning combustor, amore effective catalytic material, iron oxide, is present in flyash. While the SO

3formation in this process is important,

the experimental data is insufficient to quantify the SO3

formation.Dennis and Hayhurst [4] used an 80mm diameter

fluidized-bed combustor and mass spectrometer for measur-ing the concentration of SO

3. They confirmed the amount of

SO3formed at atmospheric pressure to be very low andmuch

less than the equilibrium concentration. The rate measured

2 ISRN Environmental Chemistry

was 100 times faster than expected for oxidation in the gasphase.

Willium and Gibbs [5] measured SO2and SO

3emissions

from a 0.3m2 fluidized-bed combustor. They reported thatthe higher the excess air level, the lower the SO

2emissions (on

removal of the dilution effect, SO2increased with an increase

in excess air, reaching a limiting value of 1300 ppm above 30%excess air), and that SO

3emissions increased slightly with the

increase in excess air. Barnes [6] also observed the similareffect of excess air on SO

2and SO

3emissions. Willium and

Gibbs [5] and Barnes [6] found that the higher the sulphurcontent of fuel, the higher the SO

2emissions, while the SO

3

was unaffected by the fuel sulphur content [5]. Coal fed tothe bed caused SO

2and SO

3emissions to increase than when

fed to the surface [5]; SO3shows a weak dependence on bed

temperature [5, 6].Barnes [6] studied the effect of sand particle size, flu-

idizing velocity, and bed depth on SO2and SO

3emissions.

Barnes’ findings indicate that fine sand (0.300mm) produceshigh SO

2and SO

3emissions. Increasing fluidizing velocity

from 1 to 2m/s caused reduced formation of SO2in the

bed and freeboard. An increase in bed depth increased SO3

emission; a deep bed (30 cm) and fine sand resulted in a slightdecrease in SO

2emission.

Oxygen availability and fluidizing characteristics withinthe bed also affect SO

3formation. Ahn et al. [7] found that

for pulverized coal, concentrations of SO3and SO

2were

significantly higher for oxy-fired conditions as comparedto air-fired conditions. In circulating fluidized bed, SO

3

concentrations were notably higher for oxy-fired conditionstoo. For higher sulfur coal, SO

3concentrations were 4–6

times greater on average.Their findings contradict the findingof Barnes [6].

Hindiyarti et al. [8] investigated the reaction of SO3with

H, O, and OH radical. The revised rate constant calculatedby them suggests that SO

3and O reaction is found to be

insignificant during most conditions. According to them,SO3+H is the major consumption reaction for SO

3.

Stanger and Wall [9] reviewed published work on SO3

concentrations and emissions under oxy-fuel firing. Theirconclusion is that the conversion of SO

2to SO

3is consider-

ably variable.Willium andGibbs [5] found that coal char had a very sig-

nificant removal effect on SO3emissions because SO

3above

the bed was 50% greater than in the exit. Barnes [6] said thatunburnt char does not have a major effect on SO

3as carbon

carryover increases under the given operating conditions.Her results showed that the quantity of inert particles (30 cmdeep bed) resulted in an increase in heterogeneous catalyticreaction of SO

2to form SO

3.

Burdett et al. [3] carried out experiments in a microbal-ance to study the effect of limestone on SO

3emissions.

According to Burdett et al. [3], the reaction of CaO, O2, and

SO2, in order to yield CaSO

4(reaction (3)), must occur in two

separate steps. Two possibilities exist, either SO2reacts with

CaO and CaSO3is formed, which is then oxidized, or else the

formation of SO3in gas phase or on a stone surface is followed

by an attack on the CaO. Consider the following:

CaO + 12O2+ SO2→ CaSO

4(3)

Route 1

CaO + SO2→ CaSO

3 (4)

CaSO3+1

2O2→ CaSO

4(5)

Route (2)

SO2+1

2O2→ SO

3(6)

SO3+ CaO → CaSO

4 (7)

It is not possible, due to the lack of available experimentaldata, to say which of these mechanisms is operative undergiven conditions, although both may be important. A mostinteresting comparison between the SO

𝑥level detected with

and without limestone (proposed by Burdett et al. [10]) isthe SO

2/SO3ratio. Without limestone, the ratio they found

was 2200/33 (or 67 : 1), and with limestone it increased to350/1.5 (or 230 : 1). It is clear that SO

3is depressed to a

greater extent than SO2on the addition of limestone, and

they attribute this to the higher reactivity of SO3compared

with that of SO2. Burdett [11] and Burdett et al. [10] have

assumed that SO2oxidizes to SO

3in the particles at a rate

dependent on local SO2and O

2concentrations, with SO

3

diffusing through theCaSO4shell and reactingwithCaO.The

rate of formation of CaSO4may be linked to different rates

of production of SO3at different locations within the stone.

At a high oxygen level, oxidation increases preferentially atthe edge of a particle. High utilization is achieved when SO

2

diffusion in the interior of the stone ismaximized, and this, inturn, implies a low SO

3formation rate. Barnes [6] reported a

decrease in SO2conversion to SO

3with oxygen concentration

and an increase with SO2concentration.

Fieldes et al. [12] reported the achievement of a highfractional sulphation (0.36) when coal is burnt in the bed.It appears that the fraction of the sulphur gas phase, whichis SO3, has a substantial effect on the fractional sulphation

of the limestone. Ash also appears to remove SO3selectively.

The mechanism of this hypothesis supposes that the directreaction of CaO with SO

3is faster than a reaction via

the CaSO3intermediate. This paper examines the factors

responsible for formation and reduction of SO2

and SO3

from a coal-fired fluidized bed under varying operatingconditions.

2. Apparatus and Procedure

The main features of the fluidized-bed combustor and ancil-laries are presented in Figure 1. The bed consisted of silicasand of mean size 0.700mm. Fluidizing air was supplied by afan and metered and introduced through a distributor plate.For staged combustion, the secondary air was introducedinto the combustor through a stainless-steel pipe 100 cmabove the bed surface. In staged combustion mode, the totalcombustion air is separated into a primary air stream supplied

ISRN Environmental Chemistry 3

Table 1: Typical analysis of Linby and Daw Mill coal.

Weight (%)Linby Daw Mill

Proximate analysis (dry basis)Ash 9.3 4.0Volatile matter 31.0 37.8Fixed carbon 59.7 58.2

Ultimate analysis (dry basis)Carbon 72.3 77.3Hydrogen 4.9 5.1Oxygen 10.17 10.44Nitrogen 1.5 1.3Sulfur 1.53 1.66Moisture 8.0 6.3Gross calorific value (mJ/kg) 30.24 31.54

Table 2: Chemical composition of Ballidone and Penrith limestone.

Ballidone PenrithCaCO3 97.8% 95.6%CaO 55.5% 52.5%CO2 (at 1000

∘C) 42.3% 43.1%SiO2 1.9% 2.8%Fe2O3 0.09% 0.4%TiO2 ⋅Al2O3 0.08% 0.8%MnO 0.3% 0.2%

to fluidize the bed and a secondary air stream injected abovethe bed to complete the combustion. For example, in 70 : 30staging, 30% of the total air is injected as secondary air.

The bed was preheated by a propane burner that was fixedabove the bed, and the fluidizing airflow rate was adjustedto the lowest level to minimize heating time. Coal was fedin the combustor when the bed temperature reached 550∘C.When the bed temperature reached 800∘C, the desired coalfeed rate was adjusted to a constant value, the propane burnerwas switched off, and the fluidizing air was adjusted to therequired level.The bed temperature was maintained constantby using an adjustable cooling coil with circulating water.Concentrations of O

2, CO, CO

2, and SO

2were recorded

continuously by an ADC-RF infrared gas analyzer.The experiments were carried out at bed temperatures

of 800–850∘C, fluidizing velocities of 1-2m/s, and excess airlevels of 0%–60%. Static bed height was 20–30 cm. Two typesof coal bituminous Linby and Daw Mill of 3–16mm (large)diameter in size and two types of limestone, Ballidone andPenrith of


Screw feederL.S. hopper

Secondary air

Rotary valve

Plenum

D.C. motor

Primary airPG

Blower

Air

Hot vertical probeTC 5

Sample probeCyclone

TC 6

Stack

Propane

Burner

Air

TC 4

TC 3

TC 2TC 1

Ash and spentsorbentcollector

Coal hopper

N2

PG = pressure gauge

TC = thermocouple

CWinCWout

Figure 1: Main features of fluidized-bed combustor and ancillaries.

121110

987654

0 1 2 3 4 5 6 7 8 9 10 11 12

SO3

emiss

ion

(ppm

)

Oxygen concentration in the fuel gas (%)

Fluidizing velocity 1.0 m/s, 30 cm bed heightFluidizing velocity 1.5 m/s, 20 cm bed heightFluidizing velocity 2.0 m/s, 20 cm bed height

Figure 2: Influence of fluidizing velocity, excess air, and bedheight on SO

3emissions during unstaged combustion at 850∘C bed

temperature and coarse sand (corrected to 5% in flue equivalent).

proposed in which O2and SO

2competitively chemisorb on

the surface, and the rate of reaction is controlled by gas-phasemolecule of SO

2reacting with adsorbed O atom.

Willium and Gibbs [5] have tested many coal types forSO3concentration without limestone in 750–900∘C temper-

ature range. Their findings suggest that ash (having tracesof Ca, Mg, Na, K, etc.) is the principle removing species ofSO3. In another experiment, when pure SO

2was introduced,

the SO3reacted with added char at 850∘C in the absence of

oxygen to give SO3of 7 vpm in the outlet, which suggests that

char is important in the removal of SO3. He also observed a

50% reduction in SO3in the freeboard.According toWillium,

the reduction was due to the reaction of SO3with unburnt

char.SO3emissions are dependent on the oxygen and sulfur

dioxide concentrations and were found to follow a similartrend. Willium and Gibbs [5] found that in contrast tothe effect on SO

2emissions, fine coal produced lower SO

3

emissions. In this study, SO3emissions were slightly higher

when fine sand was used and tended to increase with bedheight. This suggests that unburnt char does not have asignificant effect on SO

3emissions. The results of this study

indicate that the amount of particles in the bed could have asignificant effect on SO

3emissions, resulting in an increase

in the heterogeneous catalytic reaction of SO2to form SO

3

as the quantity of bed particles increases. Higher bed height,therefore, will also result in high SO

3emissions. The oxygen

concentration and fluidizing velocity will also affect SO3

formation.

4.3. SO3Emissions with Limestone under Unstaged Combus-

tion. SO3emissions decrease in the presence of limestone

and the reduction is temperature sensitive. The SO3reduc-

tions were less sensitive than the reductions achieved for SO2

at similar conditions [15, 16]. At a temperature around 850∘C,the SO

3reductions were only 28% of the SO

2reductions, but


30272421181512

9630

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

SO3

redu

ctio

n (%

)

Ca/S molar ratio

10% excess air30% excess air48% excess air

Figure 3: Influence of limestone addition on SO3reduction during

unstaged combustion 25 cmheight, 1.0m/s fluidizing velocity, 850∘Cbed temperature, and 12 cm limestone addition height.

322824201612

840

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

SO3

redu

ctio

n (%

)

Ca/S molar ratio

40% excess air60% excess air80% excess air

Figure 4: Influence of limestone addition on SO3reduction during

unstaged combustion 25 cmheight, 1.0m/s fluidizing velocity, 850∘Cbed temperature, and 42 cm limestone addition height, as well ascoarse sand.

at 800∘C, the reductions reached 70% of the SO2reduction

level. The results corresponding to the operating conditionsare shown graphically in Figures 3 and 4. It should be notedthat the SO

3reduction shown in Figure 3 was obtained

when limestone was injected 12 cm above the distributor, andFigure 4 represents the results when limestone was injected42 cm above the distributor.

At a temperature of 850∘C, some of the SO2will always be

converted to SO3via reaction (6). The conversion decreases

with oxygen concentration and increases with sulfur dioxideconcentration. An increase in temperature enhances the rateof SO3formation. SO

3can react with CaO to formCaSO

4via

reaction (7). The rate of this reaction is temperature depen-dent. Yilmaz et al. [17] studied the thermal dissociation of SO

3

in the range of 800–1200∘C under atmospheric pressure. Atthe location in the flame where the net SO

3formation rate is

zero, he determined a rate constant of 6.9× 1010 cm3mol−1 s−1

2422201816141210

86420

0 2 4 6 8 10 12 14 16 18

SO3

redu

ctio

n (%

)

Ca/S molar ratio

Limestone injection height 42 cm, excess air, 32%Limestone injection height 42 cm, excess air, 60%

Figure 5: Influence of limestone addition on SO3level in the flue

under staged combustion.

for SO3+ N2→ SO

3+ O + N

2; that was consistent with

other flame results. A high temperature lowers the reactionrate. Therefore, at a high temperature, more SO

3is produced

but less will be consumed in sulphation. As a result, a largerdecrease in SO

3emissions is observed at lower temperature.

The effect of excess air can also be seen in these graphs.SO3emissions have been found to increase with excess air,

but upon removing the dilution effect, the increase is withina narrow range, indicating that there could be an optimumreduction at a particular excess air beyond which the SO

3

reduction decreases. An increase in the fluidizing velocity haslittle effect on the overall reduction of SO

3emissions.

During another set of experiments, Penrith limestonewas added to the Daw Mill coal. It was observed that SO

3

emissions were decreased in the presence of limestone, andthe reduction was temperature dependent. At the highertemperature of 850∘C, the SO

3reductions were 18%–20% of

the SO2reduction, but at 800∘C the SO

3reductions reached

55% of the SO2reduction level. Figure 5 shows the results of

this set.

4.4. Comparison to Reported Work (Conducted on Microbal-ance or Small Bed of 36–78mm ID). Burdett et al. [3] havereported that the reaction between limestone and sulfuroxides is highly sensitive to changes in O

2, SO, and SO

3

concentrations. Absorption of SO3by the coal ash cannot be

quantified on themicrobalance, and themicrobalance resultsare not applicable to fluidized combustor.

Fieldes et al. [12] have reported that extent of SO2oxida-

tion to SO3varied with SO

2and O

2concentration. The coal

combustion test showed that the lower SO3concentrations

are due to its selective removal by ash. They had tested avariety of limestone, and in all the cases the mole fraction ofCaO converted to CaSO

4was affected by inlet oxygen in the

same way as Penrith limestone.Thibault et al. [18] have conducted experiments on a

small (6mm) fixed bed packed with CaO particle. They havetested two grain size of the sorbent and reported that forefficient capture of SO

3a small grain size and openmacropore

structure are essential.


4.5. Comparison to ReportedWork (Conducted on Pilot Scale).Burdett et al. [19] have reported fractional conversion of SO

2

to SO3decreased from about 1.5% in the limestone-free case

to around 0.35% when the limestone and alkaline ash werepresent, which was due to the greater reaction of SO

3with

limestone compared with ash, the absorption occurring bothin the bed itself and in the freeboard.

Burdett et al. [10] have reported that combustion of a 3%sulfur coal in a bed burning at 900∘C generated 33 vpm ofSO3and proposed that the effect of O

2on sulphation capacity

results from the formation of SO3within the pores of the

stone.

4.6. SO3Emissions without Limestone under Staged Combus-

tion. Merryman and Levy [20] have conducted staged exper-iments on a quartz tube methane burner producing stablemethane-H

2S flame within desired fuel-air ratio without a

sorbent presence under staged combustion conditions. Theyhave reported that when the remaining excess air was injectedinto these gases, the maximum amount of SO

3formed was

greater than formed when this additional air was includedwith the initial combustion air, the overall excess of airbeing the same in both cases. The experimental conditionsof Merryman and Levy do not match with our fluidized bed;therefore, their results are not comparable with this study.

During this study, the SO3emissions under staged com-

bustion without limestone could not be monitored exten-sively due to malfunctioning of SO

3analyzer.

4.7. SO3Emissions with Limestone under Staged Combustion.

The concentration of SO3emissions at 1.5m/s and 20% excess

air was 17.0 ppm, which decreased to 5.5 ppm in the presenceof limestone. The SO

3emissions at 70/30 staged (1.5m/s,

850∘C) combustion (without limestone) were similar to thoseof unstaged combustion (without limestone). However, it wasobserved that in the presence of limestone, staged combus-tion results in a higher reduction of SO

3than unstaged.

Figure 6 gives the SO3emissions as a function of PACR. The

emissions at 15% secondary air were 1.5 ppm and increased to7.2 ppm at 45% secondary air. It is clear that SO

3is depressed

on the addition of limestone during both the unstaged andstaged operations, and the extent of reduction was higherunder staged combustion.

Figure 6 shows that the maximum removal of SO3occurs

at a lower staging levels of 85/15, and, as the bed becomesmore substoichiometric, the rate of SO

3removal decreases.

This trend indicates the formation of SO3in the freeboard

which bypasses the limestone and appears in the flue. Thisincrease in SO

3reduction with the in-bed air ratio is in

agreement with Barnes [6] findings.The results of the staged combustion test with Daw Mill

coal in the presence of Penrith limestone indicate that SO3

emissions varied little with changes in excess air. However,if excess air is coupled with fluidizing velocity, then it hadsome effects on the emissions. At higher velocity of 2m/s, thechange was up to 4 ppm.The concentration of SO

3emissions

at 1.5m/s and 30% excess air was 20 ppm which decreased to

8

6

4

2

00.6 0.8 1 1.2 1.4 1.6

20% excess air

SO3

emiss

ion

(ppm

)

Primary air to coal ratio

Figure 6: Influence of PACR on SO3emission at 1.5m/s fluidizing

velocity and coarse sand.

5

4.5

4

3.5

3

2.5

20 5 10 15 20 25 30 35

SO3

emiss

ion

(ppm

)

Secondary air (%)

Figure 7: Influence of staging SO3emission in the flue secondary

air injection height 100 cm above the distributor, excess air 30%.

8 ppm under staged combustion in the presence of limestone.The results of Daw Mill coal test are shown in Figure 7.

It should be noted that there is no published work onSO3emissions under staged combustion conditions with or

without limestone on any scale. Therefore, the results of thisstudy could not be compared.

5. Conclusion

The experimental data shows that during unstaged com-bustion without limestone, SO

3emissions are dependent

on oxygen and SO2concentration. SO

3emissions increase

slightly with excess air, reaching a limiting value, and thenslowly decrease. SO

3emissions are less sensitive to change

in bed temperature. However, the fluidizing velocity and bedheight affect the emissions.

In the presence of limestone, SO3emissions are reduced

during both staged and unstaged operations, and the reduc-tion is temperature sensitive. However, during staged com-bustion, the reduction is enhanced. As staged fluidized-bed combustion is a proven technique to reduce NO

𝑥and

SO2emissions, therefore, it should be possible to operate a

fluidized-bed combustor under a stagedmodewith limestoneto keep SO

2, SO3, and NO

𝑥emissions to a minimum.


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Research Article Emissions of SO from a Coal-Fired ...downloads.hindawi.com/journals/isrn/2013/514751.pdfHeterogeneous Catalysis of SO 2 on Bed Particles and Heat Transfer Surfaces